1. Field of the Invention
The present invention relates to an optical scanning apparatus and an image forming apparatus and, in particular, to an optical scanning apparatus which scans a surface to be scanned (i.e., a surface which is an object of scanning), and an image forming apparatus which is provided with the optical scanning apparatus.
2. Description of the Related Art
As an image forming apparatus which forms an image by using a laser beam, there is known an image forming apparatus which forms an image by: scanning the surface of a rotating photoreceptor drum with a laser beam to thereby form a latent image on the surface of the photoreceptor drum; obtaining a toner image by visualizing the latent image; and then fixing the toner image on a piece of paper as a recording medium.
Regarding this kind of image forming apparatus, there has recently been proposed an apparatus in which: a light source device, a deflector, and even a scanning lens are contained in a single core housing made, e.g., of a resin molding or an aluminum die-casting; and optical parts such as a folding mirror for folding an optical path, scanning lens and the like which are disposed between the deflector and the photoreceptor drum are contained in a sub-housing which is integrally formed with the above-described core housing (see, e.g., patent document 1) JP-A-2006-126506 and patent document 2) Japanese Patent No. 4027870).
In the apparatuses described in the above patent documents 1) and 2), even in case the distance between the adjoining photoreceptor drums may have been changed due, e.g., design changes, the core housing can still be used in common. Therefore, the period for development can be shortened and the parts can be used in common with each other, resulting in a reduction in manufacturing cost of the apparatus.
In a tandem type of color image forming apparatus (optical scanning apparatus), it is necessary to overlap the images (toner images) on the photoreceptor drums corresponding to respective colors of toners with high accuracy. For this purpose, the optical scanning apparatus must detect the scanning position (particularly, the position in the sub-scanning direction) on each of the photoreceptor drums corresponding to the respective colors, and then correct the scanning position depending on the result of detection (see, e.g., patent document 3) JP-A-2005-208513).
In general, in an optical scanning apparatus to be used in an image forming apparatus, there is disposed a synchronous detection sensor for detecting the timing of writing in the scanning region which corresponds to the image region (exposure region), on an outside of the scanning region. Therefore, as the laser beam for synchronous detection, there is used a laser beam a part of which is separated just before entering the scanning region on the surface to be scanned. The separated laser beam is incident on the synchronous detection sensor by optical element such as a mirror.
As a result, in an optical scanning apparatus in which a plurality of folding mirrors are disposed on the laser beam path from the deflector to the surface to be scanned (surface of the photoreceptor drum), there are often cases where the optical path (synchronized beam path) from the deflector to the synchronized detection sensor must be disposed on both outer sides of the folding mirrors, resulting in a great deal of restriction in mechanical layout. In particular, in case of the apparatus having two-part housing, it was difficult to secure an optical path for the synchronous beam because the folding mirrors are bridged between the opposite side plates.
In addition, the detector to detect the scanning position by the laser beam must be mounted accurately on a predetermined mounting position. For example, if the relative positional relationship between the detector and each of the optical elements varies after a lapse of time or with a change in temperature, deviation may occur in the relationship between the scanning position on the photoreceptor drum (on the surface to be scanned) and the scanning position to be detected by the detecting sensor, resulting in a disadvantage in that the scanning position cannot be detected accurately.